1. Field of the Invention
This invention relates to a method for thermal dehydration of brown coal, by which an excellent heat recovery can be accomplished.
2. Brief Description of the Prior Art
Coal has been generally used as fuels, or have been hydrogenated and fractionated into fractions suitable for various applications. In the case of certain types of coal with high moisture contents, particularly certain brown coal which normally contain about 60 percent of moisture, thermal efficiencies are detrimentally low if they are directly subjected to hydrogenation or liquefaction or used as fuels. It is, thus, necessary that such coals be dehydrated beforehand to a moisture content of about 10 percent. Heretofore, a flash drying method has been mostly employed to dry the coals. This method is such that brown coal, for example, is finely divided and exposed to a hot gas stream to evaporate the water.
In this prior art method, however, the amount of heat required to heat the gas stream to a temperature necessary for the dehydration of brown coal is as high as about 25 percent, in terms of the heat value of brown coal based on the amount of brown coal that is treated. Since this means that a considerable additional amount of brown coal is inevitably consumed, the method is quite disadvantageous costwise. Generally, in this type of drying method, the hot gas stream is made by the combustion of an air and fuel mixtures. In this case, the excess oxygen in the hot gas stream oxidizes the brown coal grains to produce an oxide film on the surface. When the slurry is subjected to hydrogenation or liquefaction, this oxide film considerably interferes with the reaction. Furthermore, because the temperature of the hot gas stream is as high as about 900.degree. C., the volatile components in the brown coal are partially decomposed, gasified and carried away by the hot gas stream, thus resulting in reduced products yielded in the reaction.